An easy way to get this achievement quicker than the other solutions:
Start up a new game on the lowest difficulty in the new episode, "No Rest for the Living". Literally right infront of you as you start the first level is a berserk pack - hidden behind a silver UAC wall. As Catatonic Nali points out below, you don't need to get the kills while the screen is red as the pack increases your punching power for the rest of that level. Simply obtain 20 fist kills after picking up the pack in that level, and achievement unlocked.
Credit for the idea goes to Shaun2k4's achievement guide on x360a:
http://www.xbox360achievements.org/forum/showthread.php?t=23...